Wasn't sure if I should post this in diet or Workout forum, but here goes...

I searched the forum for an answer, but didn't find anything in the first 200 threads made on cardio, so I apologize if this has been answered before (or recnetly).

I recently began doing cardio in the pm, but I was wondering how long I need to fast beforehand to burn fat efficiently and not calories from my previous meal. I usually workout early afternoon (around 4) and have a protein/dextrose shake afterwards. Then, about 3hours later I do cardio for around 30min at 60-70% of my mhr. Would the carbs I took earlier hinder my fat loss? I'm concerrned I'm may be burning my pwo carbs instead of fat. Would the 3 hours be enough time? If not, how long should I wait after a pro/carb meal? Thanks.